1. Field of the Invention
The present invention relates to an image forming apparatus that involves an electrostatic copying process for image forming (e.g., a copier, facsimile machine, and printer), a process cartridge provided to the image forming apparatus, and an image forming method adopted for the image forming apparatus.
2. Description of the Related Art
In recent years, information processing systems that utilize electrophotography have been remarkably developed. In particular, photo-printers that covert information into digital signals for information recording by means of light beam have been remarkably successful in terms of their print quality and reliability. This digital recording technology is applied not only to printers, but also to general copiers, and therefore, the development of so-called digital copiers has been made. In addition, since analogue copiers that utilize this digital recording technology are provided with various information processing functions, it is expected that the demand for such copiers will increase in the near future.
In such digital copiers and digital printers, an image generally occupies as low as 10% or less of the area of document, and therefore, reversal development (positive/negative development) is mainly employed in which image portions are exposed and charge-removed portions of the photoconductor are developed using toner in view of degradation of the light source and photo fatigue of photoconductor. The reversal development requires less amount of irradiation for the photoconductor and is advantageous in light of photo fatigue due to repetitive exposure. The reversal development, however, entails generation of dot defects (e.g., background smear and black dots) at backgrounds (or white solid regions) in a case of charge leakage caused due to minute defects on the photoconductor. These unwanted dots may be mistaken for dots in a drawing or for periods and/or commas in an English document; therefore, they are fatal defects for images.
In most cases, these defects are mainly attributed to partial charge leakage from the photoconductor. Major challenges to this problem include improving the pressure resistance of the photoconductor, improving the uniformity of charge distribution over the photoconductor surface, and keeping voltage more constant. To achieve these objects, an attempt has been made to provide an intermediate layer (under layer) between a conductive support and a photosensitive layer. This intermediate layer is composed primarily of binder resin, and contains dispersed filler on an as-needed basis.
For example, if such an intermediate layer consists of binder resin as disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 47-6341, 60-66258, 52-10138 and 58-105155, the intermediate layer needs to be made very thin because binder resin is highly electrically insulating; in most cases, the intermediate layer needs to be as thin as 2 μm or less. In this case, the intermediate layer is produced through wet coating process and thus it is difficult to avoid generation of pinholes in the film being made. For this reason, the intermediate layer may not be as effective as expected.
Because almost all mainstream photoconductors are hole transporting photoconductors, their intermediate layers are made thick by the addition of electron transporting fillers to thereby prevent the generation of pinholes therein.
Intermediate layers composed of dispersed filler resin as disclosed in JP-A Nos. 58-58556, 60-111255, 59-17557, 60-32054, 64-68762, 64-68763, 64-73352, 64-73353, 1-118848 and 1-118849, however, contain ultrafine filler particles of submicron or smaller sizes, which are too costly and bulky to be used. For these reasons, submicron particles (primary particle diameter of 0.3 μm at the minimum) are often used. Such submicron particles entail re-aggregation of filler particles in their dispersion or upon deposition of film to result in film thickness variations of 1 μm or greater in some cases, making it impossible to provide an intermediate layer that allows designing of a photoconductor with uniform charge distribution and constant voltage.
In recent years the exposure density has increased and toner particles for development have become finer and finer, and the resolution of image has increased accordingly. Moreover, color-image printing has become available and therefore the opportunity to printout solid images and half-tone images has also increased. Under such circumstances, how electrostatic characteristics of the photoconductor can be made constant even after repetitive use and/or under different usage environments is a great challenge to be tackled. Much attempts have been made, though, a satisfactory technology has yet been made.
When a photoconductor is used time and time again, a surface layer (generally a charge transporting layer) of the photoconductor wears out. General image forming apparatus that use a photoconductor apply constant voltage for the charging of the photoconductor (i.e., non-exposed portions). Accordingly, when the surface layer wears out, the intensity of electric field applied to the photoconductor increases. As described above, since image defects such as background smear are caused due primarily to charge leakage, the likelihood of charge leakage is dependent on the electric field intensity; the higher the intensity, the more likely it is that charge leakage occurs.
An attempt has been made to provide a surface protection layer on the photoconductor surface in order to solve this problem. Examples of photosensitive layers with improved wear resistance include (i) those using a curable binder for a crosslinked charge transporting layer (see JP-A No. 56-48637), (ii) those using a high-molecular charge transporting substance (see JP-A No. 64-1728), and (iii) those in which an inorganic filler is dispersed in a crosslinked charge transporting layer (see JP-A No. 04-281461). A photoconductor is also known that contains a cured material resulted from polyfunctional acrylate monomers for improved wear resistance and improvided scratch resistance of the photosensitive layers (i) (see Japanese Patent (JP-B) No. 3262488). Moreover, it is also known to provide an charge transporting layer formed from a coating solution consisting of monomers containing a carbon-carbon double bond, a charge transporting substance containing a carbon-carbon double bond, and a binder resin (see JP-B No. 3194392). Furthermore, photosensitive layers that contain compounds obtained by curing hole transporting compounds bearing two or more polymerizable groups in one molecule are also known (see JP-A No. 2000-66425).
These surface protection technologies have allowed improvement of wear resistance of the photoconductor and lowered the increase rate of electrical field intensity, thereby reducing in fact the generation of image defects due to charge leakage. However, problems specific to the protection layer have occurred. For example, the amount of substance that accumulates on the surface of the photoconductor after repetitive use increases due to the reduced wear volume of the surface protective layer, resulting in creation of abnormal images (e.g., blurred images). These problems, however, can be solved by improving the protective layer and/or by using different methods of using the photoconductor, e.g., by employing a drum heater.
The improvements for a surface protective layer and developments for the method of aptly using a photoconductor have increased the wear resistance of the photoconductor, apparently extending the photoconductor life. Thus, a long time usage of photoconductor has been made possible, which seems to be impossible with photoconductor having no surface protective layers. Electrostatic fatigue has unexpectedly become a life-determining factor of photoconductor in terms of image defects caused due to photoconductor wearing out and charge leakage.